Realistically

This app is all you need for a quick demo of a riff or any song idea in your head.
I dont know about anyone else, but personally I actually like the natural lo-fi effect the mic gives your voice. Honestly the only things wrong with the app is that when you want to re record a track, you have to start from the beginning as opposed to a specific spot, and obviously that there are only 4 tracks, but its a lot better than only 2! But, its free. Thats the best part. The is probably the only app Id use for recording ideas for songs.

The Flinns about Multi Track Song Recorder